1. Field of the Invention
The present invention relates to an internal combustion engine with a variable compression ratio, as well as to a corresponding compression ratio control method.
2. Description of the Related Art
Diverse internal combustion engines with a function of variable compression ratio have been proposed recently. The high setting of the compression ratio ensures efficient power generation but tends to cause knocking. The compression ratio is thus varied according to the driving conditions. While the internal combustion engine has a low load, the potential for the knocking is low and the compression ratio is set to a large value. While the internal combustion engine has a high load, on the other hand, the potential for the knocking is high and the compression ratio is set to a small value.
A proposed compression ratio varying mechanism makes a crank casing for supporting a crankshaft and a cylinder block of a piston head apart from each other and close to each other to vary the compression ratio (for example, see Patent Document 1).
Patent Document 1: Japanese Patent Laid-Open Gazette No. 7-26981
In this cited Patent Document 1, an eccentric cam shaft is interposed between the two mechanical members, that is, the crank casing and the cylinder block, and a worm and a worm wheel are used to transmit the power to the eccentric cam shaft. The worm is linked with a driving source, such as a motor, whereas the worm wheel is linked with the object of actuation (that is, the eccentric cam shaft). Rotations of the motor in a normal direction and in an inverse direction rotate the eccentric cam shaft to make the two mechanical members apart from each other and close to each other.
In this prior art variable compression ratio engine, combustion pressure generated in a combustion chamber works to make the relative position of the piston to the cylinder, that is, the relative position of the crank casing to the cylinder block, apart from each other. The force due to the combustion pressure (hereafter referred to as the force of the combustion pressure) accordingly works to supplement the driving force required by the compression ratio varying mechanism in the case of decreasing the compression ratio. In the case of increasing the compression ratio, on the other hand, the force of the combustion pressure works to interfere with actuation of the compression ratio varying mechanism. In this case, it is required to actuate the compression ratio varying mechanism against the combustion pressure. Transmission of a large driving force to the compression ratio varying mechanism is essential in this case. Namely the driving force to be transmitted to the compression ratio varying mechanism in the case of decreasing the compression ratio is different from the required driving force in the case of increasing the compression ratio. The driving source is thus required to have high power performance, which ensures generation of a maximum required driving force in the course of a variation in compression ratio.
In the course of decreasing the compression ratio, the engine has a high load. A slow decrease of the compression ratio thus heightens the potential for knocking. A quick decrease of the compression ratio is required to prevent the occurrence of knocking. The driving source is accordingly required to have a high response and rotating characteristics in a wide range of revolution speed, in addition to the extremely high power performance. This undesirably increases the size of the driving source and thereby the size of the whole engine including the compression ratio varying mechanism, while making control of the driving source rather complicated.
In the mechanism of changing the positional relation between the mechanical members with rotation of the eccentric cam shaft to vary the compression ratio, the compression ratio depends upon the engagement of the eccentric cams with their mating elements, that is, the rotational position of the eccentric cam shaft. The force of the combustion pressure acts on the eccentric cam shaft to assist or interfere with the driving force of the driving source. The rotational position of the eccentric cam shaft affects application of the force due to the combustion pressure onto the eccentric cam shaft (that is, the magnitude of the force to rotate the eccentric cam shaft).
In the course of varying the compression ratio, there are a frictional force due to the rotation of the eccentric cam shaft and a frictional force due to the positional change of the mechanical members. These frictional forces act to interfere with transmission of the driving force from the driving source. Even when the force of the combustion pressure works to supplement the driving force of the driving source in the case of decreasing the compression ratio, the frictional forces may reduce or even totally cancel the supplementary action in a range of low compression ratio. The driving force is thus required to have the performance to allow a decrease in compression ratio without any supplementary force of the combustion pressure. This undesirably increases the size of the driving source.